In recent times, hops have been subjected to extraction predominantly with dichloromethane (methylene chloride) in order to obtain the bitter principle, and the hops thus treated are then treated with hot water in a second extraction step in order to obtain a so-called tannin extract. The two extracts are mixed in a predetermined ratio in accordance with the consumer's wishes. Methylene chloride has the advantage that it has very good solvent properties for the bitter principle in hops, is not combustible, is immiscible in water and can be distilled off at a low temperature.
Hexane and methanol also continue to be used as extracting agents to a certain extent.
For technical and commercial reasons, however, it is difficult to remove the extracting agent from the extract itself in great quantity. For example, a residual solvent content of 2.2% by weight of methylene chloride in the hops extract is officially allowed.
Despite its low toxicity, methylene chloride has quite recently become somewhat suspect as an extracting agent for food and snack items, especially in the case of decaffeinated coffee and soybean oil, so it is not impossible that its use may soon no longer be permitted as an extracting agent in foodstuffs.
All other solvents suitable for hops extraction are relatively highly toxic and suffer from the same difficulties as methylene chloride. For instance, ether and alcohol were long ago recommended for specialized hops extraction processes, without ever becoming widely accepted. According to German Patent No. 54 812, dating from 1890, the lupulone is first separated out from the hops flowers, whereupon the flowers, now substantially free of lupulone, are extracted by boiling in water and the lupulone is extracted with ether or with a 10/90 mixture of ether/alcohol. The two extracts are then condensed and then mixed together. According to German Patent No. 535 841, from 1928, oils and resins are extracted from the hops by passing water and alcohol, partially as steam and partially as condensate, in circles through the hops in a closed system in a vacuum, in a sort of circulation evaporator. This process, and particularly the apparatus required for it, are much too complicated and expensive, however, to have any commercial significance. The first extraction process to attain commercial and technological success was that using methylene chloride, at the beginning of the 1920's. In comparison, yields when hexane or methanol is used as an extracting agent are not very satisfactory.
In the search for new solvents for use in hops extraction, ethyl alcohol was not considered useful for various reasons, although it is present in trace quantities in the human organism as a product of metabolism, and although residual ethanol which may be contained in a hops extract is not objectionable as a product-specific component in beer. The characteristics of ethyl alcohol which were heretofore considered disadvantageous for hops extraction were its combustibility and the associated danger of explosion, for instance, as well as its high boiling point, high heat of vaporization and its miscibility with water. A further disadvantage was seen in the fact that with ethanol, especially aqueous ethanol, substances other than bitter principle and above all polyphenols (tannins) are extracted as well in various quantities, making it difficult to standardize the extract in terms of a certain bitterness content and preventing the production of a pure-resin extract by a single extraction process, as is frequently desired.
This is also confirmed by further prior art. For instance, German Patent No. 635 624 proposes that hops undergo a first extraction with ether or--only in the case of hops low in hops oil--with alcohol, and the remaining draff then undergoes extraction with an alcohol/water mixture, preferably after an intervening extraction using a mixture of ether and alcohol and perhaps water. This process was never put into actual practice, precisely because it was not possible, with alcohol, to obtain a pure bitter-principle extract. Although not specifically indicated in the patent, conventional 96% alcohol must have been used, which must be painstakingly fractionated again and again during the course of the process so as to be able to recover the hops oil as well. For this reason again, this process is not suited to the production of an extract which can be set at a desired standard, because when working with substantially water-free alcohol, there is no balance attained between the extracted and the recovered hops oil.
German Patent No. 737 809 begins with hops extract, with the intention of making a standardizable preparation from it, and this extract is decomposed either with an organic solvent for the hops resins or with water, so as to extract the hops tannin, and the components of the extract, thus separated, can then be mixed together in a desired ratio. Thus it is necessary first to produce a hops extract (for instance, by the old method of German Patent No. 635 624), by a two-step extraction with ether or alcohol, and then to mix the components thus obtained so as to produce a standard preparation. The much older German Patent No. 395 502, referring to the method of German Patent No. 54 812 cited above, discloses the extraction first of the hops substantially freed of lupulone and then of the lupulone itself, first with ether and then in a second step with alcohol; the alcohol extract is then separated into soft resin and hard resin by further extraction with methyl ether, and the residue from the ether extraction and the alcohol extraction is then subjected to a third extraction with water. Thus, as in German Patent No. 54 812, a mechanical removal of the lupulone from the hops is performed, and then a total of four extracts is obtained; when these four extracts are mixed together to attain a standardized preparation, hops oil from other sources must then be added as well.
German laid-open application DE-OS No. 14 42 160 also begins with hops extract, which then undergoes further extraction; the extract used as raw material can be extracted with alcohol at a strength of from 80 to 90% by volume, in order to obtain an extract rich in soft resins from an extract originally obtained with benzene. Again, this is not a process for the extraction of hops but rather a purification process for raw extract, for the purpose of further processing to produce isomerized extracts.